1. Field of the Invention
The present invention relates to the preparation of linear esters by the carbonylation of monoolefinic compounds, namely, by reacting carbon monoxide and an alcohol with compounds containing but a single olefinic double bond.
The invention more specifically relates to the preparation of alkyl adipates by the carbonylation of alkyl pentenoates.
2. Description of the Prior Art
It is well known to this art, from Bulletin of the Chemical Society of Japan, 46, pages 526 and 527 (1973), that a mixture containing dialkyl esters and, in particular, an alkyl adipate is obtained by reacting carbon monoxide and an alcohol with an alkyl pent-3-enoate, under high pressure and at high temperature, in the presence of cobalt carbonyl and a heterocyclic aromatic nitrogen base. However, the industrial-scale development of a technique of this type, the value of which is not contested in principle, is greatly jeopardized not only by reason of the poor efficacy of the catalyst system, but also by reason of the considerable proportion of alkyl pentanoate formed, even though the reaction is carried out in the absence of hydrogen.
Furthermore, those skilled in this art are well aware that the presence of small amounts of hydrogen in the reaction medium tends to increase the efficacy of cobalt-based catalysts in processes for the synthesis of esters by reacting an alcohol and carbon monoxide with an olefinic compound.
It has nevertheless also been found that, in the majority of cases, the aforenoted favorable effect associated with the presence of small amounts of hydrogen is accompanied by an adverse influence on the process selectivity in respect of linear esters, these being the specific target products.
In fact, it is observed that the presence of hydrogen not only tends to increase the proportion of hydrogenation products in the reaction mixture, but is also capable of reducing the proportion of linear ester in the esters formed.
This adverse effect severely adversely affects the economics of these processes insofar as the utilization of the branched esters and the hydrogenation products is uncertain or even non-existent. This is the case, in particular, of the branched diesters and the alkyl pentanoates produced during the carbonylation of alkyl pentenoates. Indeed, as these products are in practice destroyed, their formation amounts to an intolerable loss of starting material. Furthermore, hydrogen can be formed in situ from the traces of water which may be present in technical-grade reactants, according to the well-known reaction: EQU H.sub.2 O+CO.fwdarw.CO.sub.2 +H.sub.2
For obvious economic reasons, it would be desirable to be able to use technical-grade carbon monoxide containing hydrogen, without this detracting from process selectivity in respect of linear esters, these being the target esters. It would also be desirable, for the same reasons, to be able to use reactants containing trace amounts of water, without this leading to a loss of starting material.